This invention relates generally to pressurized screening of a fibrous material/liquid suspension and more particularly to pressurized screening of papermaking pulps.
Paper quality is directly determined by the quality of pulp used to make that paper. Characteristics effecting pulp quality include the type or source of pulp, the uniformity of the pulp fibers, the amount of foreign matter included in the pulp, and the completeness of fiber separation achieved during initial defibering such as is achieved by chemical digestion, mechanical pulping, or recycled paper pulping techniques. Pulp quality may be enhanced by screening to remove foreign matter, dirt, and groups of unseparated fibers.
A typical pulp screening device has a housing with a generally cylindrical shape into which the pulp suspension is fed. Within the housing and radially separated therefrom, is an annular screen within which, in turn, a rotor is generally coaxially mounted. The axis of the device is most commonly vertical although many screens have a horizontal axis. Between the rotor and the screen is a gap through which the feed suspension is axially passed for screening. Usually, the top of the screen is open while the top of the rotor is closed. The rotor is commonly driven from the bottom in order to impart a circular motion to the incoming pulp suspension. As the suspension passes through the gap between the rotating rotor and the stationary screen, it is subjected to a large number of hydrodynamic displacements which are caused by protrusions and/or depressions on the surface of the rotor. The resultant pressure pulses and turbulence help to break up fiber agglomerations (flocs) and to thereby improve screening efficiency. Also, because of the alternating high and low pressure pulses, there is a significant reduction of the tendency for blockage of the screen apertures by fiber agglomerations.
As the fiber suspension travels along the length of the screen, it thickens progressively due to the extraction of liquid along with the accepts fibers. If this thickening and accompanying floc formation becomes too pronounced it can plug the screen and prevent further screening operation. This thickening tendency, therefore, limits the axial length of screen apparatus which can be employed.
One approach to counteracting the thickening tendency has been to introduce dilution liquid at or near the area of the screen at which thickening begins to hamper the screening operation. This is usually in the vicinity of the midpoint of the screen length. Introduction of dilution liquid causes increased power consumption due to the necessity for accelerating the dilution liquid in the direction of rotor travel.
In general, it is desired to have the largest screening capacity per unit possible in order to provide simple screening systems which utilize the fewest screening units. Increases of capacity attained by increasing the diameter of the apparatus are limited due to the nonlinear increase of cost of manufacturing as the diameter is increased. Increases in capacity achieved by increasing the axial length of the screen and rotor are limited by the thickening tendency as the suspension passes along the screen.
In order to attain the maximum length for the screen and rotor while maintaining the screening efficiency, it is necessary to reduce or prevent the extraction of liquid along with the accepts fibers, or to provide sufficient dilution liquid to maintain the suspension consistency at a relatively constant level throughout the entire screening process. If dilution liquid is supplied, it must be supplied in such a way as to minimize the power consumption increase related thereto, otherwise, the power cost penalty may exceed the economic advantage of the higher capacity realized by utilizing a longer screen and rotor.
In fibrous material/liquid suspensions, there are a small number of stones, uncooked chips, woody chunks, or other foreign materials of varying sizes. In many cases, the sizes of these tramp materials are such that they will not pass through the screening chamber. Instead, they wedge between the rotor and screen where they can cause severe wear and damage and also inhibit the screening action.
Other factors which may reduce screening efficiency include failure to completely break up flocs, flocs formed in feed suspensions and damage to the rotor and/or screenplate by tramp materials.
The foregoing illustrates limitations known to exist in present pulp screening systems. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.